My invention relates generally to corrosion monitoring and, more specifically, to a corrosion monitoring apparatus for providing an estimate of the remaining service life of a structure before the onset of galvanic corrosion.
Metallic structural members of aircraft, ships and other equipment are subject to galvanic corrosion in the presence of moisture. Galvanic corrosion occurs as a result of current generated by the electrochemical reaction between dissimilar metals in the presence of an electrolyte. Furthermore, certain metal alloys are susceptible to corrosion because they comprise two or more metals.
Inspection for corrosion damage is difficult because corrosion frequently occurs in inaccessible areas of the structure. Aircraft wings often cannot be nondestructively inspected because the skin is bonded to the interior support structure. Furthermore, visual inspection techniques cannot precisely estimate the remaining service life of a structure known to have corrosion damage.
Dissimilar metals in aircraft and other equipment are typically insulated from contact with one another by a moisture barrier. In an aircraft wing, for example, the skin is insulated from the interior structure. During manufacturing, the interior structure is heat-treated to remove moisture prior to being insulated and sealed by bonding the skin to it. However, imperfections or punctures in the skin or insulation can allow moisture to provide a current path between the metals. This current causes corrosion of the metals over a period of time.
Practitioners in the art have developed systems for monitoring the progress of corrosion. U.S. Pat. No. 4,380,763 issued to Peart et al. discloses a system having galvanic sensors that are instead into a corrosive environment, such as in the inaccessible recesses of an aircraft frame. Each sensor of Peart et al. comprises anode and cathode elements made of dissimilar metals. The sensor generates a detectable current when exposed to moisture. Peart et al. disclose an integrating means for measuring the cumulative current flow, which is representative of the extent of corrosion. Peart et al. disclose that the sensor metals must be identical to the metals used in the structure to obtain an accurate indication of corrosion. If the metals are not identical or the sensor is not exposed to precisely the same corrosive environment as the structure, the measurements will be inaccurate. Furthermore, the measuring device of Peart et al. must store the value representing cumulative current flow over the service life of the structure being monitored. Peart et al. use a nonvolatile memory and complex support circuitry for integrating and storing the cumulative current flow. The system is completely useless in the event that this value is lost.
U.S. Pat. No. 4,839,580 issued to Moore et al. discloses a correction monitoring sensor that utilizes the principle that the resistance of the sensor element acting as the galvanic anode increases as the anode element itself corrodes. The sensors of Moore et al. comprise a stainless steel frame having sensor portions and reference portions, each plated with a layer of metal. Corrosion monitoring systems based upon anode resistance are simpler, more reliable, and more accurate than those based upon cumulative current flow. Furthermore, sensor elements used in these systems need not be constructed of the same metals used in the structure being monitored because the rate of change in sensor element resistance is proportional to the corrosion rate of the structure.
The sensitivity of anode resistance sensors to incremental resistance change is inversely proportional to anode thickness. While a very thin anode element provides a large change in resistance in response to a small degree of corrosion, the anode must be thick enough to eliminate the need for frequent replacement. The corrosion monitoring system becomes useless if the anode element corrodes through, resulting in an open circuit. The sensor of Moore et al. is designed for continuously monitoring a corrosive environment by measuring the resistance change of a sensor pre-plated with a corrodable metal or, conversely, for monitoring electrolytic plating processes by measuring the resistance change of a sensor as the plating metal builds up of the sensor. The sensor of Moore et al. is designed to be reusable by removing it from the corrosive environment and replating it. The need to achieve a long useful sensor life inherently sacrifices sensor sensitivity.
It is well-known that metallic structural members can be protected from galvanic corrosion by providing the structural members with an anodic material that corrodes preferentially with respect to the structure, thereby sacrificially corroding the anodic material and preserving the structural members. Practitioners in the art have protected metal structures using this principle by applying a primer coating to the portion of the metal exposed to moisture. The primer applied to aluminum structures typically contains zinc, which corrodes preferentially with respect to aluminum. Similarly, ships and heavy equipment in which additional weight is not a critical design consideration have been protected by attaching plates of preferentially corroding anodic material to the hull or other structure. The structure will begin to corrode when the anodic material is completely consumed by the electrochemical reaction.
In aircraft, however, weight considerations prohibit providing enough anodic material to completely guard against corrosion of the structural members over the service life of the aircraft. Therefore, as discussed above, practitioners in the art have guarded against corrosion by sealing areas of the aircraft structure that would be particularly susceptible to corrosion. Moisture entering these sealed, uninspectable areas may cause undetected corrosion, which if allowed to progress, could cause catastrophic structural failure.
Simply detecting a moisture intrusion into these sealed areas with a galvanic sensor provides no warning period; corrosion may begin as soon as moisture has entered. Furthermore, anodic resistance sensors such as that of Moore et al. are not well-suited for use in aircraft, which require economical monitoring of a large area with a minimum contribution to weight. A reusable sensor is unnecessary in monitoring sealed, inaccesible areas of an aircraft because the area must be opened once corrosion has been detected.
It would be desirable to provide an accurate indication of the corrosive effect on the structure of any such moisture intrusion. It would further be desirable to provide an indication of the time remaining before the onset of structural corrosion to aid in decisions concerning continued use of the aircraft or its return for depot-level maintenance. Such an aid would provide a valuable maintenance scheduling tool by reducing guesswork, resulting in greater efficiency.
There is a strongly felt need in the art for a system that provides an accurate indication of the remaining service life of a structure as well as a buffer of "temporary protection," during which period decisions regarding the scheduling of aircraft maintenance can be made. These problems and deficiencies are clearly felt in the art and are solved by my invention in the manner described below.